It has heretofore been known that the thermal stability and barrier properties of oriented blow molded containers of poly(ethylene) terephthalate are significantly increased by heat setting. Thermal stability can be of two types, namely first, thermal stability required to fill a container with hot liquids or hot semi-solids without deformation of the container and, second, thermal stability to withstand the pressures of the contents during the shelf life of the filled containers, such as in carbonated beverages, which tend to cause the container to deform or grow with time, commonly known as creep.
In hot-fill applications, the following properties are important: onset of shrinkage; top load; bulge factor; and drop impact. Most of the food products are hot-filled at 85.degree.-120.degree. C. Higher onset-of-shrinkage is required so that the containers would resist shrinkage after hot filling. In addition, filling equipment exerts loads on the containers during filling operation. Since products are filled at elevated temperatures, resistance to higher top load at the filling temperature is required. After the containers are filled, the containers tend to bulge due to the load exerted by the fluid inside the container. Improved top load and bulge factor are obtained by higher mechanical properties. In summary, for hot-fill applications, a combination of higher onset-of-shrinkage and mechanical properties are required.
For beverage applications, the following properties are important: thermal stability; top load; burst test; and drop impact. The containers are filled with carbonated liquids (at a pressure usually 4 volumes) at 40.degree. F. to room temperature. Thermal stability in the case of beverage application is a measure of growth of the container during the shelf life of the container. This is often tested by subjecting the container at 100.degree. F. for 24-48 hours. In general, higher mechanical properties are required to pass the requirements for the beverage applications.
Typical processes for heat setting are shown in U.S. Pats. Nos. 4,476,170, 4,512,948 and 4,522,779.
In U.S. Pats. Nos. 4,476,170 and 4,512,948, there is disclosed an article and a process of making an oriented and heat set blow molded container of poly(ethylene) terephthalate. In the process, a preform preheated to a temperature suitable for orientation is biaxially stretched in a blow mold and then while the hollow container is still in contact with the blow mold walls, the article is raised to a higher heat setting temperature preferably in the range of 200.degree.-250.degree. C. (except for the neck) thus heat setting the container, and while the container is still at a shrinkage resisting pressure exceeding atmospheric, it is cooled in the same mold to a temperature at which is it maintains its shape when not pressurized but not below 100.degree. C. It is also particularly disclosed that this cooling step can be done in the air outside the mold while maintaining internal pressure. According to these patents, when the heat setting temperature of the hot mold ranges from 220.degree.-250.degree. C. and the quenching temperature is not below 100.degree. C., higher onset-of-shrinkage temperatures are obtained. Where quenching is performed in the hot mold, the cycle time is necessarily increased because of the necessity of heating, cooling, and reheating the mold. In addition, the molds are more complex and greater energy is required for heating and cooling. Where cooling is achieved outside of the mold while maintaining internal pressure, added cycle time is required for cooling at ambient temperature. In addition, material distribution may be adversely affected because the container is unconfined during the cooling. In a special embodiment where the cooling step is effected outside the mold, the cooling under the shrinkage resisting pressure is below 100.degree. C., even down to room temperature and lower, before the shrinkage resisting pressure is released form the hollow container, but the maximum benefit of higher onset-of-shrinkage temperature is not realized.
In U.S. Pat. No. 4,522,779, there are disclosed improved plastic containers and a process for their production. In a first embodiment, a container is blow molded in a first hot blow mold, then reblown to a larger size in a second cold mold of larger volume than the first hot mold. Such containers are stated as having improved mechanical properties, particularly very high hoop yield stresses. The subsequent quenching of an article in the larger cold mold causes the stresses induced during the reblowing in the larger cold mold to be frozen. As a result, the onset-of-shrinkage temperature is reduced because the frozen-in stresses will be relaxed at lower temperatures, as contrasted to a container that does not have frozen-in stresses. In a second embodiment, a container is blow molded in a hot blow mold, then reblown to a larger size in a second hot blow mold where it is blown to the confines of of the second mold and the container is then removed from the second hot mold and transferred to a third cold mold and cooled to room temperature while maintaining internal pressure. This would substantially increase the overall cycle time. Such a method used commercially would involve increased capital investments, complex machinery and greater operating costs. In a further embodiment, the container is blow molded in a first hot mold, reblown in a second hot mold, and thereafter the second mold is cooled to cool the container. This would substantially increase the cycle time since the second mold must be cycled between hot and cold temperatures which requires substantial time.
U.S. Pat. No. 4,385,089 (British Patent Specification No. 1,604,203) is directed to heat set biaxially oriented hollow articles and states that the preform or parison should be heated at least to biaxially oriented temperature and maintained in closed contact with a hot mold which is at a temperature of up to 40.degree. C. above the minimum orientation temperature. In one embodiment, the resultant molded hollow article is moderately cooled, causing a temperature drop of 10.degree.-30.degree. C., by introducing cooling vapor or mist into the hollow article, interrupting the cooling vapor and opening the mold. In another embodiment, the heat set article is allowed to shrink freely and then reblown in the same hot mold or in a separate cooled mold. The heat setting is 130.degree. C. or less. As a result, a lower onset-of-shrinkage temperature will be obtained. Furthermore, where the heat set article is permitted to shrink freely before being reblown, there would be a loss of mechanical properties, difficulty in obtaining proper material distribution in the reblown article and increased cycle time involved in reblowing.
Accordingly, among the objectives of the present invention are to provide an improved method for making biaxially oriented heat set poly(ethylene) terephthalate containers which have onset-of-shrinkage and mechanical properties as required for hot-fill applications and, at the same time, a method which provides substantially lower cycle times.
In accordance with the invention, the method comprises engaging the open end of a plastic parison which is at a temperature within its molecular orientation temperature range, enclosing a hot mold about the hot parison, the mold being at heat setting temperature, expanding the plastic parison within the hot mold by internal pressurization to induce biaxial orientation of the plastic parison and force the plastic parison into intimate contact and conformance with the hot mold and to maintain contact by such internal pressurization between the mold and the biaxially oriented container for a time sufficient to induce partial crystallization in the biaxially oriented container, maintaining a lower internal pressurization of the container to prevent significant shrinkage, opening the hot mold while maintaining engagement of the open end of the blown hollow container, enclosing the blown hollow container in a cold mold having substantially the same volume as the hot mold, or smaller, increasing the pressurization to force the container into intimate contact with the cold mold to cool the container while maintaining internal pressurization and then exhausting the pressure and opening the cold mold. The method results in a thermally stable container.